Analysis of W-183 and O-17 NMR chemical shifts in polyoxometalates by extended Huckel MO calculations

摘要

The extended Huckel molecular orbital (EHMO) calculations have been carried out using cluster approach to polyoxo anions, i.e. calculations have been done for a single octahedron MO6 of different symmetry and results have been used to analyze W-183 and O-17 NMR spectra. Using five d-->d* energy differences for the individual WO6 the sum Sigma 1/(E-d-->E-d*) have been calculated and plotted against the W-183 chemical shift (from +258 to -670 ppm) for corresponding type of tungsten atom and practically a linear correlation between two parameters have been observed. This points out the electronic nature of the W-183 chemical shift. Similar correlation have been found for the O-17 NMR chemical shifts (- 90 to + 800 ppm) when the plotted against product of the R-3 Sigma 1/(E-p-->E-p*) where the R-3 bond length of the corresponding W-O bond. increasing the W-183 nuclear magnetic shielding with the calculated electron population on tungsten atom for closely related anions has been observed, but no general tendency between delta and the calculated electronic charge if the symmetry of polyhedron is changed is expected. (C) 1998 Elsevier Science Ltd. All rights reserved. [References: 56]